Convenience provided by their mobility has brought wireless communication terminals that can be used in wireless LAN (local area network) and the like into wide use not only in enterprises but also in home consumer market rapidly in recent years.
Among standards for wireless LAN, a DSSS (direct sequence spread spectrum) method is defined in IEEE 802.11b; an OFDM (orthogonal frequency division multiplexing) method is defined in IEEE 802.11a; and a method for supporting both DSSS and OFDM is defined in IEEE 802.11g. At present, products that conform to the IEEE 802.11n draft standard that aims at further increase in throughput are becoming common.
In recent years, more and more various mobile terminals, such as notebook PCs (personal computers) and cellular phones, incorporate wireless LAN functionality. Many of such products available on the market include two or more antennas and employ what is called a selection diversity technique of carrying out communications by using an antenna that has a stronger received signal power. Application of this selection diversity technique is not limited to wireless LAN, and this technique is broadly used in the field of mobile wireless communication, such as cellular phone systems.
With the selection diversity technique in the field of mobile wireless communication, an antenna that has received a received signal with strongest signal power among a plurality of antennas is selected and used in signal receiving. In wireless LAN, two antennas are used, and received power of a preamble signal of each of the two antennas is measured for each packet. An antenna for use in receiving the packet is selected based on a result of the measurement.
FIG. 6 illustrates access points 100 and 200 (hereinafter, simply referred to as “AP 100” and “AP 200”), which are wireless LAN access points in infrastructure mode, a wireless communication terminal 50 that is currently communicating with the AP 100, and a wireless communication terminal 51 that is currently communicating with the AP 200.
A range where radio waves originated from an antenna of the AP 100 is indicated by a dotted line in FIG. 6. As is illustrated, the wireless communication terminal 50 is capable of receiving a packet transmitted from the AP 100 by using an antenna 60 but unable to receive the same by using an antenna 61.
When the wireless communication terminal 50 continues to receive packets by using the antenna 60, the wireless communication terminal 50 can successfully detect and receive packets originated from the AP 100; however, if the wireless communication terminal 50 uses the antenna 61 in packet receiving, the wireless communication terminal 50 cannot detect a packet originated from the AP 100 and hence fails to receive the packet. In other words, in the vicinity of a boundary of the communication range where radio waves originated from the AP 100 can reach, in some cases the wireless communication terminal successfully detects a packet but in other cases the wireless communication terminal fails to detect the same depending on an antenna that has been selected as a stand-by antenna while waiting for receiving a signal.
For instance, assume that the wireless communication terminal 50 is to select a stand-by antenna for use in receiving a packet to be received next while receiving a packet. Further assume that, selecting the antenna 60 for the AP 100 results in stronger received power than that by using the antenna 61; in contrast, selecting the antenna 61 for the wireless communication terminal 51 and for the AP 200 results in stronger received power than that by using the antenna 60 as illustrated in FIG. 6. It is assumed that under such a condition, the APs 1 and 2 and the wireless communication terminals 50 and 51 included in the wireless LAN transmit packets as illustrated in FIG. 7.
Referring to FIG. 7, the AP 100 transmits a DATA packet to the wireless communication terminal 50 first, and the wireless communication terminal 50 transmits to the AP 100 an ACK packet that acknowledges successful receipt of the DATA packet transmitted from the AP 100. Thereafter, transmission of a DATA packet from the wireless communication terminal 51, transmission of an ACK packet from the AP 200, and transmission of a DATA packet from the AP 100 are performed.
Upon unintentionally receiving an ACK packet transmitted from the AP 200, the wireless communication terminal 50 selects the antenna 61 based on a result of judgment that, because receiving power received by the antenna 61 is stronger than that received by the antenna 60, the antenna 61 is an optimum receiving antenna, causing the antenna 61 to remain to be selected as the stand-by antenna.
Accordingly, the wireless communication terminal 50 cannot detect a DATA packet to be transmitted next from the AP 100 and hence fails to receive the DATA packet. Thus, with such conventional technique as discussed above, there can be cases where an optimum stand-by antenna cannot be selected depending on a communication condition of other AP and/or wireless communication terminal.
To circumvent such a situation, techniques of causing, by software control, a wireless communication terminal to select an optimum stand-by antenna by sequentially switching antennas during a packet waiting time with a relatively small circuit size have been devised (see, for instance, Japanese Patent Application Laid-open No. 2005-252825).
Other techniques of causing a wireless communication terminal to select a stand-by antenna while receiving a packet, of which source address is a currently-communicating AP and of which destination address is not that of the wireless communication terminal, to thereby select an optimum stand-by antenna have been devised (see, for instance, Japanese Patent Application Laid-open No. 2007-143090).
However, APs and wireless communications terminals are provided close to one another in recent wireless LAN environment, causing radio waves to be crowded with one another in some cases, resulting in considerably short packet waiting time. In such a case, because the conventional wireless communication terminal described in Japanese Patent Application Laid-open No. 2005-252825 selects a stand-by antenna with software that is not suitable for high-speed processing, there can be some cases where operation of selecting a stand-by antenna lags behind high-speed processing. Put another way, the conventional wireless communication terminal described in Japanese Patent Application Laid-open No. 2005-252825 is disadvantageous in being influenced by a communication condition of another wireless communication terminal when selecting a stand-by antenna.
The conventional wireless communication terminal described in Japanese Patent Application Laid-open No. 2007-143090 is allowed to select an optimum stand-by antenna only when an AP, with which the wireless communication terminal is communicating, is communicating with another wireless communication terminal and the other wireless communication terminal is transmitting/receiving a packet. Put another way, the conventional wireless communication terminal described in Japanese Patent Application Laid-open No. 2007-143090 is disadvantageous in that the wireless communication terminal can be influenced by communication condition of another wireless communication terminal when selecting a stand-by antenna.
The wireless communication terminal described in Japanese Patent Application Laid-open No. 2007-143090 is also disadvantageous in that because the wireless communication terminal performs signal processing, such as receiving, also in a period during which a packet, of which destination address is not that of the wireless communication terminal, is transmitted, electric power is consumed in performing signal processing for such a packet.
The present invention has been conceived in view of the above circumstances and aims at providing a wireless communication terminal that is capable of selecting an optimum stand-by antenna with lower electric power consumption than that of conventional wireless communication terminals and less influenced by a communication condition of another wireless communication terminal.